Electromagnetic systems for biosensors are used in combination with magnetic beads, which commonly are solved in the biomaterial to be analysed.
This use of magnetic labels in biosensors has several advantages. One can actuate the beads by applying a magnetic field gradient. This allows to control the assay, to speed up the procedure, and to make the assay more specific and more reliable.
Furthermore, no magnetic background signal is present from biological fluids, which is an advantage when the labels are detected by magnetic means.
Magnetic labels can be detected by various means, e.g. the sensor or biosensor can be any suitable sensor to detect the presence of magnetic particles on or near to a sensor surface, based on any property of the particles, e.g. it can detect via magnetic methods, e.g. magneto-resistive methods, hall-sensors, coils, optical methods, imaging, fluorescence, chemiluminescence, absorption, scattering, surface plasmon resonance, Raman, sonic detection, e.g. surface-acoustic-wave, bulk acoustic wave, cantilever, quartz crystal etc, electrical detection, e.g. conduction, impedance, amperometric, redox cycling, etc.
The challenge is to design a magnetic system with the following properties:                high switchable gradient        magnet on only one side of the cartridge        no mechanical movement needed to achieve switching of the magnetic field        self-alignment of the cartridge in the reader, i.e. alignment of the sensor surface with respect to the maximum force region of the magnet.        
The actuation of magnetic labels requires a switchable field gradient, for forces toward and forces away from the sensor surface. These forces are needed for up-concentration of labels near to the sensor surface and for magnetic stringency, for example. A known solution is to take two magnets on either side of the cartridge [e.g. Luxton, Anal. Chem. 76, 1715 (2004)]. A disadvantage of this solution is that geometrical constraints exist for the cartridge and also for the sensor read-head, and it involves a high number of parts.